--
--  Copyright (C) 2022-2024, Free Software Foundation, Inc.
--
--  SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
--

with Ada.Containers.Generic_Array_Sort;
with Ada.Unchecked_Deallocation;

with System; use type System.Address;

package body SPARK.Containers.Formal.Unbounded_Vectors with
  SPARK_Mode => Off
is

   subtype Int is Long_Long_Integer;

   --  The minimum size of the underlying array to avoid too much resizing

   Min_Size : constant Capacity_Range :=
     Count_Type'Min (100, Capacity_Range'Last);

   -----------------------
   -- Local_Subprograms --
   -----------------------

   function To_Array_Index (Index : Index_Type) return Array_Index;

   procedure Free_Element is new Ada.Unchecked_Deallocation
     (Element_Type, Element_Access);

   procedure Free_Element_Array is new Ada.Unchecked_Deallocation
     (Element_Array, Element_Array_Access);

   procedure Insert_Space
     (Container : in out Vector;
      Before    : Extended_Index;
      Count     : Count_Type := 1);

   procedure Resize (Container : in out Vector; Size : Count_Type := 0) with
     --  Widen the sub array of the Vector

     Global => null,
     Post   => Model (Container) = Model (Container)'Old;

   ---------
   -- "=" --
   ---------

   function "=" (Left : Vector; Right : Vector) return Boolean is
   begin
      if Left'Address = Right'Address then
         return True;
      end if;

      if Length (Left) /= Length (Right) then
         return False;
      end if;

      for J in 1 .. Length (Left) loop
         if Left.Elements (J).all /= Right.Elements (J).all then
            return False;
         end if;
      end loop;

      return True;
   end "=";

   ------------
   -- Adjust --
   ------------

   procedure Adjust (V : in out Vector) is
      Dest : Element_Array_Access;
   begin
      if V.Elements = null then
         return;
      end if;

      Dest := new Element_Array (1 .. V.Elements'Length);

      for Idx in 1 .. Length (V) loop
         Dest (Idx) := new Element_Type'(V.Elements (Idx).all);
      end loop;

      V.Elements := Dest;
   end Adjust;

   ------------
   -- Append --
   ------------

   procedure Append (Container : in out Vector; New_Item : Vector) is
   begin
      if Is_Empty (New_Item) then
         return;
      end if;

      if Container.Last >= Index_Type'Last then
         raise Constraint_Error with "vector is already at its maximum length";
      end if;

      Insert (Container, Container.Last + 1, New_Item);
   end Append;

   procedure Append (Container : in out Vector; New_Item : Element_Type) is
   begin
      Append (Container, New_Item, 1);
   end Append;

   procedure Append
     (Container : in out Vector;
      New_Item  : Element_Type;
      Count     : Count_Type)
   is
   begin
      if Count = 0 then
         return;
      end if;

      if Container.Last >= Index_Type'Last then
         raise Constraint_Error with "vector is already at its maximum length";
      end if;

      Insert (Container, Container.Last + 1, New_Item, Count);
   end Append;

   ------------
   -- Assign --
   ------------

   procedure Assign (Target : in out Vector; Source : Vector) is
   begin
      if Target'Address = Source'Address then
         return;
      end if;

      Clear (Target);
      Append (Target, Source);
   end Assign;

   -----------
   -- Clear --
   -----------

   procedure Clear (Container : in out Vector) is
   begin
      for J in 1 .. Length (Container) loop
         if Container.Elements (J) /= null then
            Free_Element (Container.Elements (J));
         end if;
      end loop;
      Container.Last := No_Index;
   end Clear;

   ------------------------
   -- Constant_Reference --
   ------------------------

   function Constant_Reference
     (Container : aliased Vector;
      Index     : Index_Type) return not null access constant Element_Type
   is
   begin
      if Index > Container.Last then
         raise Constraint_Error with "Index is out of range";
      end if;

      return Container.Elements (To_Array_Index (Index));
   end Constant_Reference;

   --------------
   -- Contains --
   --------------

   function Contains
     (Container : Vector;
      Item      : Element_Type) return Boolean
   is
   begin
      return Find_Index (Container, Item) /= No_Index;
   end Contains;

   ----------
   -- Copy --
   ----------

   function Copy (Source : Vector) return Vector is
   begin
      return Target : Vector do
         Assign (Target, Source);
      end return;
   end Copy;

   ------------
   -- Delete --
   ------------

   procedure Delete (Container : in out Vector; Index : Extended_Index) is
   begin
      Delete (Container, Index, 1);
   end Delete;

   procedure Delete
     (Container : in out Vector;
      Index     : Extended_Index;
      Count     : Count_Type)
   is
      Old_Last : constant Index_Type'Base := Container.Last;
      Old_Len  : constant Count_Type := Length (Container);
      New_Last : Index_Type'Base;
      Count2   : Count_Type'Base;  -- count of items from Index to Old_Last
      Off      : Count_Type'Base;  -- Index expressed as offset from IT'First

   begin
      --  Delete removes items from the vector, the number of which is the
      --  minimum of the specified Count and the items (if any) that exist from
      --  Index to Container.Last. There are no constraints on the specified
      --  value of Count (it can be larger than what's available at this
      --  position in the vector, for example), but there are constraints on
      --  the allowed values of the Index.

      --  As a precondition on the generic actual Index_Type, the base type
      --  must include Index_Type'Pred (Index_Type'First); this is the value
      --  that Container.Last assumes when the vector is empty. However, we do
      --  not allow that as the value for Index when specifying which items
      --  should be deleted, so we must manually check. (That the user is
      --  allowed to specify the value at all here is a consequence of the
      --  declaration of the Extended_Index subtype, which includes the values
      --  in the base range that immediately precede and immediately follow the
      --  values in the Index_Type.)

      if Index < Index_Type'First then
         raise Constraint_Error with "Index is out of range (too small)";
      end if;

      --  We do allow a value greater than Container.Last to be specified as
      --  the Index, but only if it's immediately greater. This allows the
      --  corner case of deleting no items from the back end of the vector to
      --  be treated as a no-op. (It is assumed that specifying an index value
      --  greater than Last + 1 indicates some deeper flaw in the caller's
      --  algorithm, so that case is treated as a proper error.)

      if Index > Old_Last then
         if Index > Old_Last + 1 then
            raise Constraint_Error with "Index is out of range (too large)";
         end if;

         return;
      end if;

      --  Here and elsewhere we treat deleting 0 items from the container as a
      --  no-op, so we simply return.

      if Count = 0 then
         return;
      end if;

      --  We first calculate what's available for deletion starting at
      --  Index. Here and elsewhere we use the wider of Index_Type'Base and
      --  Count_Type'Base as the type for intermediate values. (See function
      --  Length for more information.)

      if Count_Type'Base'Last >= Index_Type'Pos (Index_Type'Base'Last) then
         Count2 := Count_Type'Base (Old_Last) - Count_Type'Base (Index) + 1;
      else
         Count2 := Count_Type'Base (Old_Last - Index + 1);
      end if;

      --  If more elements are requested (Count) for deletion than are
      --  available (Count2) for deletion beginning at Index, then everything
      --  from Index is deleted. There are no elements to slide down, and so
      --  all we need to do is set the value of Container.Last.

      if Count >= Count2 then

         --  The elements between Index and Container.Last must be deallocated

         for Idx in
           To_Array_Index (Index) .. To_Array_Index (Container.Last)
         loop
            Free_Element (Container.Elements (Idx));
         end loop;

         Container.Last := Index - 1;
         return;
      end if;

      --  There are some elements aren't being deleted (the requested count was
      --  less than the available count), so we must slide them down to Index.
      --  We first calculate the index values of the respective array slices,
      --  using the wider of Index_Type'Base and Count_Type'Base as the type
      --  for intermediate calculations.

      if Index_Type'Base'Last >= Count_Type'Pos (Count_Type'Last) then
         Off := Count_Type'Base (Index - Index_Type'First);
         New_Last := Old_Last - Index_Type'Base (Count);
      else
         Off := Count_Type'Base (Index) - Count_Type'Base (Index_Type'First);
         New_Last := Index_Type'Base (Count_Type'Base (Old_Last) - Count);
      end if;

      --  The array index values for each slice have already been determined,
      --  so we just slide down to Index the elements that weren't deleted.

      declare
         EA  : Element_Array_Access renames Container.Elements;
         Idx : constant Count_Type := EA'First + Off;
      begin

         --  The elements between Idx and Idx + Count - 1 must be deallocated

         for J in 0 .. Count - 1 loop
            Free_Element (Container.Elements (Idx + J));
         end loop;

         EA (Idx .. Old_Len - Count) := EA (Idx + Count .. Old_Len);
         Container.Last := New_Last;
      end;
   end Delete;

   ------------------
   -- Delete_First --
   ------------------

   procedure Delete_First (Container : in out Vector) is
   begin
      Delete_First (Container, 1);
   end Delete_First;

   procedure Delete_First (Container : in out Vector; Count : Count_Type) is
   begin
      if Count = 0 then
         return;

      elsif Count >= Length (Container) then
         Clear (Container);
         return;

      else
         Delete (Container, Index_Type'First, Count);
      end if;
   end Delete_First;

   -----------------
   -- Delete_Last --
   -----------------

   procedure Delete_Last (Container : in out Vector) is
   begin
      Delete_Last (Container, 1);
   end Delete_Last;

   procedure Delete_Last (Container : in out Vector; Count : Count_Type) is
      Index : Index_Type'Base;
   begin
      if Count = 0 then
         return;
      end if;

      --  There is no restriction on how large Count can be when deleting
      --  items. If it is equal or greater than the current length, then this
      --  is equivalent to clearing the vector. (In particular, there's no need
      --  for us to actually calculate the new value for Last.)

      --  If the requested count is less than the current length, then we must
      --  calculate the new value for Last. For the type we use the widest of
      --  Index_Type'Base and Count_Type'Base for the intermediate values of
      --  our calculation.

      if Count >= Length (Container) then
         Clear (Container);
         return;

      elsif Index_Type'Base'Last >= Count_Type'Pos (Count_Type'Last) then
         Index := Container.Last - Index_Type'Base (Count) + 1;

      else
         Index :=
           Index_Type'Base (Count_Type'Base (Container.Last) - Count + 1);
      end if;

      Delete (Container, Index, Count);
   end Delete_Last;

   -------------
   -- Element --
   -------------

   function Element
     (Container : Vector;
      Index     : Extended_Index) return Element_Type
   is
   begin
      if Index > Container.Last or Index = No_Index then
         raise Constraint_Error with "Index is out of range";
      end if;

      return Container.Elements (To_Array_Index (Index)).all;
   end Element;

   --------------
   -- Finalize --
   --------------

   procedure Finalize (V : in out Vector) is
   begin
      if V.Elements = null then
         return;
      end if;

      if V.Last /= No_Index then
         for Idx in Array_Index'First .. To_Array_Index (V.Last) loop
            Free_Element (V.Elements (Idx));
         end loop;
      end if;

      V.Last := No_Index;
      Free_Element_Array (V.Elements);
   end Finalize;

   ----------------
   -- Find_Index --
   ----------------

   function Find_Index
     (Container : Vector;
      Item      : Element_Type;
      Index     : Index_Type := Index_Type'First) return Extended_Index
   is
      K    : Count_Type;
      Last : constant Extended_Index := Last_Index (Container);

   begin
      K := To_Array_Index (Index);
      for Indx in Index .. Last loop
         if Container.Elements (K).all = Item then
            return Indx;
         end if;

         K := K + 1;
      end loop;

      return No_Index;
   end Find_Index;

   -------------------
   -- First_Element --
   -------------------

   function First_Element (Container : Vector) return Element_Type is
   begin
      if Is_Empty (Container) then
         raise Constraint_Error with "Container is empty";
      else
         return Container.Elements (1).all;
      end if;
   end First_Element;

   -----------------
   -- First_Index --
   -----------------

   function First_Index (Container : Vector) return Index_Type is
      pragma Unreferenced (Container);
   begin
      return Index_Type'First;
   end First_Index;

   ------------------
   -- Formal_Model --
   ------------------

   package body Formal_Model is

      -------------------------
      -- Element_Logic_Equal --
      -------------------------

      function Element_Logic_Equal (Left, Right : Element_Type) return Boolean
      is
      begin
         Check_Or_Fail;
         return Left = Right;
      end Element_Logic_Equal;

      -------------------------
      -- M_Elements_In_Union --
      -------------------------

      function M_Elements_In_Union
        (Container : M.Sequence;
         Left      : M.Sequence;
         Right     : M.Sequence) return Boolean
      is
      begin
         for Index in Index_Type'First .. M.Last (Container) loop
            declare
               Elem : constant Element_Type := Element (Container, Index);
               Found : Boolean := False;
            begin
               for J in Index_Type'First .. M.Last (Left) loop
                  if Element_Logic_Equal
                    (Element (Container, Index), Element (Left, J))
                  then
                     Found := True;
                     exit;
                  end if;
               end loop;

               if not Found then
                  for J in Index_Type'First .. M.Last (Right) loop
                     if Element_Logic_Equal
                       (Element (Container, Index), Element (Right, J))
                     then
                        Found := True;
                        exit;
                     end if;
                  end loop;
               end if;

               if not Found then
                  return False;
               end if;
            end;
         end loop;

         return True;
      end M_Elements_In_Union;

      -------------------------
      -- M_Elements_Included --
      -------------------------

      function M_Elements_Included
        (Left  : M.Sequence;
         L_Fst : Index_Type := Index_Type'First;
         L_Lst : Extended_Index;
         Right : M.Sequence;
         R_Fst : Index_Type := Index_Type'First;
         R_Lst : Extended_Index) return Boolean
      is
      begin
         for I in L_Fst .. L_Lst loop
            declare
               Found : Boolean := False;
               J     : Extended_Index := R_Fst - 1;

            begin
               while not Found and J < R_Lst loop
                  J := J + 1;
                  if Element_Logic_Equal
                       (Element (Left, I), Element (Right, J))
                  then
                     Found := True;
                  end if;
               end loop;

               if not Found then
                  return False;
               end if;
            end;
         end loop;

         return True;
      end M_Elements_Included;

      -------------------------
      -- M_Elements_Reversed --
      -------------------------

      function M_Elements_Reversed
        (Left  : M.Sequence;
         Right : M.Sequence) return Boolean
      is
         L : constant Index_Type := M.Last (Left);

      begin
         if L /= M.Last (Right) then
            return False;
         end if;

         for I in Index_Type'First .. L loop
            if not Element_Logic_Equal
              (Element (Left, I), Element (Right, L - I + 1))
            then
               return False;
            end if;
         end loop;

         return True;
      end M_Elements_Reversed;

      ------------------------
      -- M_Elements_Swapped --
      ------------------------

      function M_Elements_Swapped
        (Left  : M.Sequence;
         Right : M.Sequence;
         X     : Index_Type;
         Y     : Index_Type) return Boolean
      is
      begin
         if M.Length (Left) /= M.Length (Right)
           or else not Element_Logic_Equal
             (Element (Left, X), Element (Right, Y))
           or else not Element_Logic_Equal
             (Element (Left, Y), Element (Right, X))
         then
            return False;
         end if;

         for I in Index_Type'First .. M.Last (Left) loop
            if I /= X and then I /= Y
              and then not Element_Logic_Equal
                (Element (Left, I), Element (Right, I))
            then
               return False;
            end if;
         end loop;

         return True;
      end M_Elements_Swapped;

      -----------
      -- Model --
      -----------

      function Model (Container : Vector) return M.Sequence is
         R : M.Sequence;

      begin
         for Position in 1 .. Length (Container) loop
            R := M.Add (R, Container.Elements (Position).all);
         end loop;

         return R;
      end Model;

   end Formal_Model;

   ---------------------
   -- Generic_Sorting --
   ---------------------

   package body Generic_Sorting with SPARK_Mode => Off is

      ------------------
      -- Formal_Model --
      ------------------

      function Strictly_Less (L, R : Element_Access) return Boolean with
      --  Compare Element with access check

        Global => null;

      package body Formal_Model is

         -----------------------
         -- M_Elements_Sorted --
         -----------------------

         function M_Elements_Sorted (Container : M.Sequence) return Boolean is
         begin
            if M.Length (Container) = 0 then
               return True;
            end if;

            declare
               E1 : Element_Type := Element (Container, Index_Type'First);

            begin
               for I in Index_Type'First + 1 .. M.Last (Container) loop
                  declare
                     E2 : constant Element_Type := Element (Container, I);

                  begin
                     if E2 < E1 then
                        return False;
                     end if;

                     E1 := E2;
                  end;
               end loop;
            end;

            return True;
         end M_Elements_Sorted;

      end Formal_Model;

      ---------------
      -- Is_Sorted --
      ---------------

      function Is_Sorted (Container : Vector) return Boolean is
         L : constant Capacity_Range := Length (Container);

      begin
         for J in 1 .. L - 1 loop
            if Container.Elements (J + 1).all < Container.Elements (J).all then
               return False;
            end if;
         end loop;

         return True;
      end Is_Sorted;

      -----------
      -- Merge --
      -----------

      procedure Merge (Target : in out Vector; Source : in out Vector) is
         I : Count_Type;
         J : Count_Type;

      begin
         if Target'Address = Source'Address then
            raise Program_Error with "Target and Source denote same container";
         end if;

         if Length (Source) = 0 then
            return;
         end if;

         if Length (Target) = 0 then
            Move (Target => Target, Source => Source);
            return;
         end if;

         I := Length (Target);

         --  Check if the merge of Source and Target will fit in a Vector

         if Count_Type'Base (Last_Count) - I < Length (Source) then
            raise Constraint_Error with "Source too large for Target";
         end if;

         Insert_Space (Target, Target.Last + 1, Length (Source));

         declare
            TA : Element_Array_Access renames Target.Elements;
            SA : Element_Array_Access renames Source.Elements;

         begin
            J := Length (Target);
            while Length (Source) /= 0 loop
               if I = 0 then
                  TA (1 .. J) := SA (1 .. Length (Source));
                  Source.Last := No_Index;
                  exit;
               end if;
               if SA (Length (Source)).all < TA (I).all then
                  TA (J) := TA (I);
                  I := I - 1;
               else
                  TA (J) := SA (Length (Source));
                  Source.Last := Source.Last - 1;
               end if;

               J := J - 1;
            end loop;
         end;
      end Merge;

      ----------
      -- Sort --
      ----------

      procedure Sort (Container : in out Vector) is

         procedure Sort is
           new Generic_Array_Sort
                 (Index_Type   => Array_Index,
                  Element_Type => Element_Access,
                  Array_Type   => Element_Array,
                  "<"          => Strictly_Less);

         Len : constant Capacity_Range := Length (Container);

      begin
         if Container.Last <= Index_Type'First then
            return;
         else
            Sort (Container.Elements (1 .. Len));
         end if;
      end Sort;

      -------------------
      -- Strictly_Less --
      -------------------

      function Strictly_Less (L, R : Element_Access) return Boolean is
        (if L = null
         then
            R = null
         elsif R = null
         then
            L = null
         else
            L.all < R.all);

   end Generic_Sorting;

   -----------------
   -- Has_Element --
   -----------------

   function Has_Element
     (Container : Vector;
      Position  : Extended_Index) return Boolean
   is
   begin
      return Position in First_Index (Container) .. Last_Index (Container);
   end Has_Element;

   ------------
   -- Insert --
   ------------

   procedure Insert
     (Container : in out Vector;
      Before    : Extended_Index;
      New_Item  : Element_Type)
   is
   begin
      Insert (Container, Before, New_Item, 1);
   end Insert;

   procedure Insert
     (Container : in out Vector;
      Before    : Extended_Index;
      New_Item  : Element_Type;
      Count     : Count_Type)
   is
      J : Count_Type'Base;  -- scratch

   begin
      --  Use Insert_Space to create the "hole" (the destination slice)

      Insert_Space (Container, Before, Count);

      J := To_Array_Index (Before);

      Container.Elements (J .. J - 1 + Count) :=
        (others => new Element_Type'(New_Item));
   end Insert;

   procedure Insert
     (Container : in out Vector;
      Before    : Extended_Index;
      New_Item  : Vector)
   is
      N : constant Count_Type := Length (New_Item);
      B : Count_Type;  -- index Before converted to Count_Type

   begin
      if Container'Address = New_Item'Address then
         raise Program_Error with
           "Container and New_Item denote same container";
      end if;

      --  Use Insert_Space to create the "hole" (the destination slice) into
      --  which we copy the source items.

      Insert_Space (Container, Before, Count => N);

      if N = 0 then

         --  There's nothing else to do here (vetting of parameters was
         --  performed already in Insert_Space), so we simply return.

         return;
      end if;

      B := To_Array_Index (Before);

      --  Make a copy of all the elements to avoid sharing

      for J in 1 .. N loop
         Container.Elements (B - 1 + J) :=
           new Element_Type'(New_Item.Elements (J).all);
      end loop;
   end Insert;

   ------------------
   -- Insert_Space --
   ------------------

   procedure Insert_Space
     (Container : in out Vector;
      Before    : Extended_Index;
      Count     : Count_Type := 1)
   is
      Old_Length : constant Count_Type := Length (Container);

      Max_Length : Count_Type'Base;  -- determined from range of Index_Type
      New_Length : Count_Type'Base;  -- sum of current length and Count

      Index : Index_Type'Base;  -- scratch for intermediate values
      J     : Count_Type'Base;  -- scratch

   begin
      --  As a precondition on the generic actual Index_Type, the base type
      --  must include Index_Type'Pred (Index_Type'First); this is the value
      --  that Container.Last assumes when the vector is empty. However, we do
      --  not allow that as the value for Index when specifying where the new
      --  items should be inserted, so we must manually check. (That the user
      --  is allowed to specify the value at all here is a consequence of the
      --  declaration of the Extended_Index subtype, which includes the values
      --  in the base range that immediately precede and immediately follow the
      --  values in the Index_Type.)

      if Before < Index_Type'First then
         raise Constraint_Error with
           "Before index is out of range (too small)";
      end if;

      --  We do allow a value greater than Container.Last to be specified as
      --  the Index, but only if it's immediately greater. This allows for the
      --  case of appending items to the back end of the vector. (It is assumed
      --  that specifying an index value greater than Last + 1 indicates some
      --  deeper flaw in the caller's algorithm, so that case is treated as a
      --  proper error.)

      if Before > Container.Last
        and then Before - 1 > Container.Last
      then
         raise Constraint_Error with
           "Before index is out of range (too large)";
      end if;

      --  We treat inserting 0 items into the container as a no-op, so we
      --  simply return.

      if Count = 0 then
         return;
      end if;

      --  There are two constraints we need to satisfy. The first constraint is
      --  that a container cannot have more than Count_Type'Last elements, so
      --  we must check the sum of the current length and the insertion count.
      --  Note that the value cannot be simply added because the result may
      --  overflow.

      if Old_Length > Count_Type'Last - Count then
         raise Constraint_Error with "Count is out of range";
      end if;

      --  It is now safe compute the length of the new vector, without fear of
      --  overflow.

      New_Length := Old_Length + Count;

      --  The second constraint is that the new Last index value cannot exceed
      --  Index_Type'Last. In each branch below, we calculate the maximum
      --  length (computed from the range of values in Index_Type), and then
      --  compare the new length to the maximum length. If the new length is
      --  acceptable, then we compute the new last index from that.

      if Index_Type'Base'Last >= Count_Type'Pos (Count_Type'Last) then

         --  We have to handle the case when there might be more values in the
         --  range of Index_Type than in the range of Count_Type.

         if Index_Type'First <= 0 then

            --  We know that No_Index (the same as Index_Type'First - 1) is
            --  less than 0, so it is safe to compute the following sum without
            --  fear of overflow.

            Index := No_Index + Index_Type'Base (Count_Type'Last);

            if Index <= Index_Type'Last then

               --  We have determined that range of Index_Type has at least as
               --  many values as in Count_Type, so Count_Type'Last is the
               --  maximum number of items that are allowed.

               Max_Length := Count_Type'Last;

            else
               --  The range of Index_Type has fewer values than in Count_Type,
               --  so the maximum number of items is computed from the range of
               --  the Index_Type.

               Max_Length := Count_Type'Base (Index_Type'Last - No_Index);
            end if;

         else
            --  No_Index is equal or greater than 0, so we can safely compute
            --  the difference without fear of overflow (which we would have to
            --  worry about if No_Index were less than 0, but that case is
            --  handled above).

            if Index_Type'Last - No_Index >= Count_Type'Pos (Count_Type'Last)
            then
               --  We have determined that range of Index_Type has at least as
               --  many values as in Count_Type, so Count_Type'Last is the
               --  maximum number of items that are allowed.

               Max_Length := Count_Type'Last;

            else
               --  The range of Index_Type has fewer values than in Count_Type,
               --  so the maximum number of items is computed from the range of
               --  the Index_Type.

               Max_Length := Count_Type'Base (Index_Type'Last - No_Index);
            end if;
         end if;

      elsif Index_Type'First <= 0 then

         --  We know that No_Index (the same as Index_Type'First - 1) is less
         --  than 0, so it is safe to compute the following sum without fear of
         --  overflow.

         J := Count_Type'Base (No_Index) + Count_Type'Last;

         if J <= Count_Type'Base (Index_Type'Last) then

            --  We have determined that range of Index_Type has at least as
            --  many values as in Count_Type, so Count_Type'Last is the maximum
            --  number of items that are allowed.

            Max_Length := Count_Type'Last;

         else
            --  The range of Index_Type has fewer values than Count_Type does,
            --  so the maximum number of items is computed from the range of
            --  the Index_Type.

            Max_Length :=
              Count_Type'Base (Index_Type'Last) - Count_Type'Base (No_Index);
         end if;

      else
         --  No_Index is equal or greater than 0, so we can safely compute the
         --  difference without fear of overflow (which we would have to worry
         --  about if No_Index were less than 0, but that case is handled
         --  above).

         Max_Length :=
           Count_Type'Base (Index_Type'Last) - Count_Type'Base (No_Index);
      end if;

      --  We have just computed the maximum length (number of items). We must
      --  now compare the requested length to the maximum length, as we do not
      --  allow a vector expand beyond the maximum (because that would create
      --  an internal array with a last index value greater than
      --  Index_Type'Last, with no way to index those elements).

      if New_Length > Max_Length then
         raise Constraint_Error with "Count is out of range";

      --  Raise Capacity_Error if the new length exceeds the container's
      --  capacity.

      elsif New_Length > Last_Count then
         raise Capacity_Error with "New length is larger than capacity";
      end if;

      J := To_Array_Index (Before);

      declare
         EA : Element_Array_Access renames Container.Elements;
      begin
         if Container.Elements = null
              or else Container.Elements'Length < New_Length
         then

            --  The array must be resized

            Resize (Container, New_Length);
         end if;

         if Before <= Container.Last then

            --  The new items are being inserted before some existing
            --  elements, so we must slide the existing elements up to their
            --  new home.

            EA (J + Count .. New_Length) := EA (J .. Old_Length);
            EA (J .. J + Count - 1) := (others => null);
         end if;
      end;

      if Index_Type'Base'Last >= Count_Type'Pos (Count_Type'Last) then
         Container.Last := No_Index + Index_Type'Base (New_Length);

      else
         Container.Last :=
           Index_Type'Base (Count_Type'Base (No_Index) + New_Length);
      end if;
   end Insert_Space;

   --------------
   -- Is_Empty --
   --------------

   function Is_Empty (Container : Vector) return Boolean is
   begin
      return Last_Index (Container) < Index_Type'First;
   end Is_Empty;

   ------------------
   -- Last_Element --
   ------------------

   function Last_Element (Container : Vector) return Element_Type is
   begin
      if Is_Empty (Container) then
         raise Constraint_Error with "Container is empty";
      else
         return Container.Elements (Length (Container)).all;
      end if;
   end Last_Element;

   ----------------
   -- Last_Index --
   ----------------

   function Last_Index (Container : Vector) return Extended_Index is
   begin
      return Container.Last;
   end Last_Index;

   ------------
   -- Length --
   ------------

   function Length (Container : Vector) return Capacity_Range is
      L : constant Int := Int (Container.Last);
      F : constant Int := Int (Index_Type'First);
      N : constant Int'Base := L - F + 1;

   begin
      return Capacity_Range (N);
   end Length;

   ----------
   -- Move --
   ----------

   procedure Move (Target : in out Vector; Source : in out Vector) is
   begin
      if Target'Address = Source'Address then
         return;
      end if;

      if Target.Elements /= null then
         Clear (Target);
         Free_Element_Array (Target.Elements);
      end if;

      Target.Elements := Source.Elements;
      Target.Last := Source.Last;

      Source.Elements := null;
      Source.Last := No_Index;
   end Move;

   ------------
   -- Prepend --
   ------------

   procedure Prepend (Container : in out Vector; New_Item : Vector) is
   begin
      Insert (Container, Index_Type'First, New_Item);
   end Prepend;

   procedure Prepend (Container : in out Vector; New_Item : Element_Type) is
   begin
      Prepend (Container, New_Item, 1);
   end Prepend;

   procedure Prepend
     (Container : in out Vector;
      New_Item  : Element_Type;
      Count     : Count_Type)
   is
   begin
      Insert (Container, Index_Type'First, New_Item, Count);
   end Prepend;

   ---------------
   -- Reference --
   ---------------

   function Reference
     (Container : not null access Vector;
      Index     : Index_Type) return not null access Element_Type
   is
   begin
      if Index > Container.Last then
         raise Constraint_Error with "Index is out of range";
      end if;

      return Container.Elements (To_Array_Index (Index));
   end Reference;

   ---------------------
   -- Replace_Element --
   ---------------------

   procedure Replace_Element
     (Container : in out Vector;
      Index     : Index_Type;
      New_Item  : Element_Type)
   is
      Index_Arr : constant Array_Index := To_Array_Index (Index);
   begin
      if Index > Container.Last then
         raise Constraint_Error with "Index is out of range";
      end if;

      Free_Element (Container.Elements (Index_Arr));

      Container.Elements (Index_Arr) := new Element_Type'(New_Item);
   end Replace_Element;

   ------------
   -- Resize --
   ------------

   procedure Resize (Container : in out Vector; Size : Count_Type := 0)
   is
      EA : Element_Array_Access renames Container.Elements;

   begin
      if EA = null then
         EA := new Element_Array (1 .. Count_Type'Max (Min_Size, Size));
         Container.Last := No_Index;
         return;
      end if;

      declare
         New_Length : constant Capacity_Range :=
           (if Container.Elements'Length < Capacity_Range'Last / 2
            then (if Container.Elements'Length * 2 < Size
                  then Size
                  else Container.Elements'Length * 2)
            else Capacity_Range'Last);

         New_Array  : constant Element_Array_Access :=
           new Element_Array (1 .. Count_Type'Max (Min_Size, New_Length));
         Last       : constant Count_Type := Length (Container);

      begin
         New_Array (1 .. Last) := EA (1 .. Last);
         Free_Element_Array (EA);
         EA := New_Array;
      end;
   end Resize;

   ----------------------
   -- Reverse_Elements --
   ----------------------

   procedure Reverse_Elements (Container : in out Vector) is
   begin
      if Length (Container) <= 1 then
         return;
      end if;

      declare
         I, J : Capacity_Range;
         E    : Element_Array_Access renames Container.Elements;
      begin
         I := 1;
         J := Length (Container);
         while I < J loop
            declare
               EI : constant Element_Access := E (I);

            begin
               E (I) := E (J);
               E (J) := EI;
            end;

            I := I + 1;
            J := J - 1;
         end loop;
      end;
   end Reverse_Elements;

   ------------------------
   -- Reverse_Find_Index --
   ------------------------

   function Reverse_Find_Index
     (Container : Vector;
      Item      : Element_Type;
      Index     : Index_Type := Index_Type'Last) return Extended_Index
   is
      Last : Index_Type'Base;
      K    : Count_Type'Base;

   begin
      if Is_Empty (Container) then
         return No_Index;
      end if;

      if Index > Last_Index (Container) then
         Last := Last_Index (Container);
      else
         Last := Index;
      end if;

      K := To_Array_Index (Last);
      for Indx in reverse Index_Type'First .. Last loop
         if Container.Elements (K).all = Item then
            return Indx;
         end if;

         K := K - 1;
      end loop;

      return No_Index;
   end Reverse_Find_Index;

   ----------
   -- Swap --
   ----------

   procedure Swap
     (Container : in out Vector;
      I         : Index_Type;
      J         : Index_Type)
   is
   begin
      if I > Container.Last then
         raise Constraint_Error with "I index is out of range";
      end if;

      if J > Container.Last then
         raise Constraint_Error with "J index is out of range";
      end if;

      if I = J then
         return;
      end if;

      declare
         II : constant Array_Index := To_Array_Index (I);
         JJ : constant Array_Index := To_Array_Index (J);

         EI : Element_Access renames Container.Elements (II);
         EJ : Element_Access renames Container.Elements (JJ);

         EI_Copy : constant Element_Access := EI;

      begin
         EI := EJ;
         EJ := EI_Copy;
      end;
   end Swap;

   --------------------
   -- To_Array_Index --
   --------------------

   function To_Array_Index (Index : Index_Type) return Array_Index is
      Offset : Count_Type'Base;

   begin
      --  We know that
      --    Index >= Index_Type'First
      --  hence we also know that
      --    Index - Index_Type'First >= 0

      --  The issue is that even though 0 is guaranteed to be a value in
      --  the type Index_Type'Base, there's no guarantee that the difference
      --  is a value in that type. To prevent overflow we use the wider
      --  of Count_Type'Base and Index_Type'Base to perform intermediate
      --  calculations.

      if Index_Type'Base'Last >= Count_Type'Pos (Count_Type'Last) then
         Offset := Count_Type'Base (Index - Index_Type'First);

      else
         Offset :=
           Count_Type'Base (Index) - Count_Type'Base (Index_Type'First);
      end if;

      --  The array index subtype for all container element arrays always
      --  starts with 1.

      return 1 + Offset;
   end To_Array_Index;

   ---------------
   -- To_Vector --
   ---------------

   function To_Vector
     (New_Item : Element_Type;
      Length   : Capacity_Range) return Vector
   is
   begin
      if Length = 0 then
         return Empty_Vector;
      end if;

      declare
         Vec : Vector;

      begin
         Append (Vec, New_Item, Length);

         return Vec;
      end;
   end To_Vector;

end SPARK.Containers.Formal.Unbounded_Vectors;